Aluminum treating process



2,993,847 ALUMINUM TREATING PROCESS Joseph A. Poch, Detroit, Mich.,assignor to Burroughs Corporation, Detroit, Mich., a corporation ofMichigan No Drawing. Filed Apr. 4, 1958, Ser. No. 726,310 1 Claim. (Cl.20415) This invention relates generally to a method of treating thesurface of metallic objects and particularly to a method of treating thesurface of aluminum products.

In the manufacture of certain products of aluminum, it is required thatthe surface be treated to protect it against corrosion and that acertain area or spot on the surface be electrically conductive for thepurpose of attaching thereto an electrical connector, such as a groundconnection. The preferred protection against corrosion of an aluminumproduct is acquired by anodizing the surface thereof, but, as is wellknown, the anodized finish provides a very poor electrical conductor. Asa result, it has been the practice to anodize the entire surface of analuminum product and then grind off the anodized finish in a selectedarea or spot and daub this spot with any of the well known acidicchromate conversion solutions that is both corrosion resistant and has asatisfactorily high electrical conductivity. This method achieved theresult, but the steps of grinding and daubing are both cumbersome andexpensive and it has been found that when the solution is daubed ontothe area or spot, following the anodizing step, that the solutionpenetrated the anodized aluminum with resultant chemical action thatdiscolored the finished anodized surface.

Accordingly, it is the principal object of the invention to provide animproved aluminum product treating method which obviates the abovementioned objections.

More specifically, it is an object of the invention to provide animproved method of treating the surface of an aluminum product to obtaina relatively small area of the surface electrically conductive and theentire surface corrosion resistant by a particular order of method stepsthat avoid interaction of the corrosion resistant and electricalconductor materials and accompanying discoloration of the surface of theproduct.

A further object of the invention is to provide an improved method oftreating the surface of an aluminum product to obtain a relatively smallarea thereof electrically conductive and the entire area corrosionresistant by a particular order of method steps that eliminates theaforementioned grinding operation and at the same time generallyfacilitates the treating of the product.

Other objects of the invention will become apparent from the followingdetailed description of the invention.

In carrying out the process of the present invention, the first step isto apply to a cleaned surface of the aluminum object, a coating of acomposition which is both corrosion resistant and electricallyconductive. Preferably this is accomplished by an acidic chromateconversion treatment of the surface of the aluminum object, as suchtreatments are extremely simple to perform and are very inexpensive. Asis well known, the application of an acidic chromate conversiontreatment to an aluminum surface converts a thin layer of the aluminumsurface by chemical reaction into a non-metallic form to produce aprotective, electrical conductive film. Specifically, the conversiontreatment may be effected by using a hexavalent chromium compound(chromic acid or a bichromate), and activators having a pH in the rangeof about 2. The solution may contain of from 1 to 4 oz. of the compoundper gallon of water. This aqueous solution may be daubed or brushed ontothe surface of the aluminum object to be treated, but preferably, theobject is immersed into a bath of the solution to facilitate nitedStates Patent production. Immersion time may vary from about 5 secondsto about 6 minutes. The object is then preferably rinsed in cold Waterand dried at a temperature preferably of about degrees Fahrenheit.

After the treated object is dry, a portion of the surface thereof,selected for attachment of an electrical connector, is covered andprotected against removal by a mask. The mask may be a suitable adhesivetape or paint, or any other suitable masking material that is alkali andacid resistant and will withstand a temperature of 150 F. The remainder,or unmasked film is then removed from the surface of the objectpreferably by submerging the object in a caustic bath, such as, sodiumhydroxide. As is well known, the sodium hydroxide will attack thealuminum and destroy the adhesion between the aluminum and the chromateto quickly strip the chromate from the product. With the mask still inplace, the cleaned product is then anodized by the well-knownelectrochemical process to impart to the surface of the aluminum adecorative protective coating except, of course, for the area or spotbeneath the mask. Finally, the mask is removed to expose thenon-anodized, electrically conductive area to which an electricalconnector, such as a ground connection, may be subsequently attached.

Any suitable chromate conversion solution may be used that is bothcorrosion resistant and electrically conductive, such as an aqueoussolution of a chromic compound selected from the group consisting ofchromic acid and water-soluble salts thereof, and ferricyanic orferrocyanic acid and water-soluble salts thereof, and mixtures of thesame. The application of such a solution to an aluminum surface resultsin a thin layer of the surface metal being converted by chemicalreaction into a nonmetallic form or amorphous mixed metallic oxidesurface coating that is both protective and electrically conductive.Below are two examples of a group of solutions that may be used with thepresent method:

enhances material handling and facilitates production by eliminating theneed of grinding away a spot on a completely anodized surface in orderto provide electrical conductive surface for aground connection. Inaddition, by first applying the electrical conductive coating ofchromate to the product, and then anodizing the remainder of the surfaceof the product, except for the masked spot, I avoid the above mentionedchemical reaction and resultant discoloration that occurs if the surfaceis first anodized and chromate then daubed onto an exposed spot of theporous aluminum surface.

What is claimed is:

The method of treating the surface of an aluminum object to protect theentire surface of the aluminum against corrosion and render a portion ofthe surface conductive for an electrical connection, which methodcomprises first applying a chromate conversion coating to the entiresurface of the object to form an electrically conductive mixed metallicoxide surface coating integral with the aluminum surface, said mixedmetallic oxide surface coating being formed by contact between theentire surface of the aluminum and an aqueous solution including ahexavalent chromium compound and having a pH of approximately 0.8 to 2,thereafter forming an alkali and acid resist mask on a portion of theelectrically conductive metallic oxide surface coating, removing theunmasked mixed metallic oxide surface coating from the aluminum surfaceby submerging the object in a caustic bath of sodium hydroxide,anodizing the unmasked aluminum surface of the object, and finallyremoving the mask.

References Cited in the file of this patent UNITED STATES PATENTS2,506,164 Morse May 2,

5 2,604,418 Stern July 22, 2,764,537 Bunch et a1. Sept. 25, 2,812,295Patrick Nov. 5, 2,885,273 Oelgoetz et a1. May 5,

FOREIGN PATENTS 735,819 Great Britain Aug. 31, 661,273 Great BritainNov. 21,

OTHER REFERENCES Metal Industry (London), Nov. 10, 1944, pages 15 293.

